The present invention relates to a surface measurement apparatus suitable for evaluating crystal defects of a semiconductor wafer and detecting extraneous particles attached on a surface of the semiconductor wafer, and particularly to a surface measurement apparatus suitable for measuring a surface state of a sample (wafer) with a high precision.
With increase in the degree of integration of an LSI (large-scale integration) circuit, there arises a serious problem in that the yield of conforming items and the reliability of MOS (metal oxide semiconductor) transistors composing the LSI circuit are decreased due to failures in the transistors. As a cause of the MOS transistor failures, typical problems are dielectric breakdown in a gate oxide film and an excessive leakage current in a junction. Most of the MOS transistor failures are caused directly or indirectly by crystal defects in a silicon substrate. That is, when a crystal defect exists in a surface area of the silicon substrate to be converted to a silicon oxide film by oxidization in an LSI circuit manufacturing process, a structural defect is formed in the silicon film to cause dielectric breakdown when the LSI circuit is operated.
Further, when a crystal defect exists in a depletion layer of a junction, the leakage current becomes larger. As described above, it is not preferable that a crystal defect is formed in a surface area in a silicon wafer where an element is to be formed because a MOS transistor failure will be caused.
Therefore, defect measurement of a silicone substrate is important in the quality control of silicon crystal.
In regard to the method of measuring such a defect, there is a technology described in a paper by Takeda in "Applied Physics", Vol. 65, No. 11 (1996), page 1162. In the method, two light beams having wavelengths of which penetration depths to a solid are different from each other by three times or more are irradiated from oblique incident light irradiating optical systems onto a wafer surface, and the intensity of the scattered light from a crystal defect is detected from a direction normal to the wafer surface. In the measuring method employing such an oblique incident light irradiating optical system, when the irradiating beam size is finely focused and the wafer adhered onto a wafer chuck is warped, the position irradiated by the beam is vertically changed according to change in the position of the wafer surface. The position irradiated by the oblique incident beam is moved on the sample surface in a direction parallel to the sample surface due to the vertical change. Supposing the irradiating beam is irradiated at the Brewster angle (75.degree.) of silicon and the height of the wafer surface is changed by 0.5 micrometers due to the reason described above, the position irradiated by the beam is changed on the wafer surface by approximately 1.9 micrometers. At the same time, the intensity of the scattered light cannot be accurately measured because the detecting position of the detection system is vertically changed due to the vertical change of the wafer surface. Therefore, it is necessary to maintain the relative position between the irradiated area and the detecting system area constant and to maintain the distance between them and the sample surface.
Therefore, it is necessary that a wafer chick having a highly flat mounting surface is used and a wafer is forcibly laid along the mounting surface of the wafer chuck by adhering the back surface of the wafer to the mounting surface using a vacuum force or an electrostatic force to adjust flatness of the wafer surface so as to become flat even if the wafer is warped.
However, in the supporting method of adjusting flatness of the wafer surface so as to become flat by adhering the back surface of the wafer to the mounting surface of the wafer chuck using a vacuum force or an electrostatic force, when a gap between the outer peripheral edge of the wafer and the mounting surface of the wafer chuck is formed, the outer peripheral edge portion of the wafer is bent in that portion and accordingly the gap cannot be formed. Therefore, it becomes difficult to insert a pair of tweezers for detaching the wafer or a transfer hand for automatically transfer the wafer between the wafer and the wafer chuck. Particularly, this problem becomes clearer in a case of employing a wafer chuck usable to plural kinds of wafers having different diameters. This is because when a wafer having a diameter smaller than a maximum diameter which the wafer chuck can handle is mounted on the wafer chuck, the mounting area of the wafer chuck becomes larger than the diameter of the mounted wafer.
Therefore, when the wafer is detached from the wafer chuck, a wafer lifting mechanism for lifting off the wafer from the wafer chuck is required.
In order to perform measurement over the whole surface of a wafer by scanning a finely focused spot of irradiating light over the wafer surface, a crystal defect measurement apparatus is designed so that the wafer is linearly moved in the radial direction while being rotated using a stage having shafts movable in the radial direction and the rotating direction of the wafer. Therefore, the wafer lifting mechanism used in such a crystal defect measurement apparatus is on the premise that it is combined with a rotating stage.
As an example of the conventional wafer lifting mechanism, a technology used in a rotating stage is disclosed in Japanese Patent Application Laid-Open No.7-249559. The mechanism comprises a rotatable stage which adheres and supports only the central portion of a wafer, and a transfer hand having U-shaped top ends or a transfer hand adhering the wafer at the peripheral portion off the central portion of the wafer is combined with a vertical moving mechanism to lift the wafer off from the wafer chuck of the rotating stage.
As another example of the conventional wafer lifting mechanism, a technology used in a positing stage of an exposure apparatus is disclosed in Japanese Patent Application Laid-Open No.8-88265. The mechanism comprises an XY stage movable in two axial directions intersecting each other at right angle, and the wafer lifting mechanism is mounted on the XY stage mechanism. When a wafer is moved along these shafts, the wafer lifting mechanism is moved together with the wafer chuck.
The conventional wafer lifting mechanism used in such a rotating stage is very difficult to insert the transfer hand into the back surface of the wafer in a case of using the wafer chuck adhering the almost whole outer peripheral edge of the back surface of the wafer such as in the crystal defect measurement apparatus, and particularly in a case where the diameter of the mounting surface of the wafer chuck is larger than the diameter of the wafer to be adhered and mounted.
Further, in a case of employing the rotating stage, the wafer lifting mechanism is difficult to be integrally constructed with the chuck rotating mechanism because the volume and the weight of the rotating portion must be reduced, because the rotating unbalance in respect to the rotating shaft must be decreased, and because in an rotating stage using a power source such as electricity or pressurized gas, the wiring or the piping is installed in the inside of the rotating shaft so that the rotating shaft may be rotated.